The present invention relates to image forming equipment having an image carrier and a developer carrier arranged face-to-face and forming an AC-superposed DC electric field between them for developing a latent image electrostatically formed on the image carrier. Also, the present invention is concerned with image forming equipment having sensor means located to face the surface of an image carrier in a position downstream of a developing device for developing an electrostatic latent image formed on the image carrier with respect to an intended direction of movement of the surface of the image carrier. The sensor means is responsive to a potential of and a reflection from a portion of the surface of the image carrier which does not positively deposit a toner, so that image forming conditions may be controlled on the basis of the outputs of the sensor means.
An electrophotographic copier, facsimile transceiver, printer or similar image forming equipment are provided with some sensors to have the image forming conditions thereof adequately controlled. The sensors include a potential sensor and an optical sensor located to face the surface of an image carrier, e.g., a photoconductive element, and a toner concentration sensor and a piezoelectric sensor disposed in a developing device. The potential sensor senses the potential of a latent image electrostatically formed on the photoconductive element, allowing the amount of charge, the amount of light for exposure and other image forming conditions to be controlled in response to the output thereof. The optical sensor is responsive to the amount of toner deposited on a pattern formed on the photoconductive element, so that the bias for development may be controlled on the basis of the output thereof. The piezoelectric sensor is responsive to the amount of developer existing in a developing device to allow a developer to be supplied in a controlled amount to the developing device. Further, the toner concentration sensor determines the concentration of a toner in the developer, i.e., the mixture ratio of toner and carrier, allowing the amount of toner in the developer to be controlled. The sensors, however, cannot operate with sufficiently high accuracy and even produce erroneous outputs.
It is a common practice with the above-described type of image forming equipment to use an AC-superposed DC bias for the development of a latent image. The bias including an AC component allows the toner to behave actively to thereby promote easy development. This not only reduces the required relative speed of the image carrier, e.g., photoconductive drum and the image carrier, e.g., developing roller but also promotes uniform development by eliminating an excessive edge effect. However, the problem with the Ac-superposed DC bias is that the previously stated sensors, particularly piezoelectric sensor and toner concentration sensor associated with the developing device, are apt to pick up noise ascribable to the AC component and cause needless carrier and toner particles to deposit on the image photoconductive drum in the event of sensing operation. The needless carrier and toner particles are wasteful and, in addition, apt to damage the photoconductive drum. To eliminate this problem, the AC component may be interrupted at the time when the sensors operate, as proposed in Japanese Patent Laid-Open Publication No. 85557/1990 by way of example. Although this kind of implementation successfully prevents the sensors from picking up noise ascribable to the AC component, it cannot prevent the needless carrier and toner particles from depositing on the photoconductive drum.